I. Field of the Invention
The present invention relates to a digital filter and, more particularly, to a digital filter used for a digital recording/reproducing apparatus.
II. Description of the prior art
Recently, a digital recording/reproducing apparatus such as a digital audio tape (DAT) recorder has been developed. In such an apparatus, as shown in FIG. 1, in a recording mode, L- and R-channel analog audio signals S.sub.L and S.sub.R respectively supplied to input terminals 1 and 2 are converted into digital audio signals by a common A/D converter 5 to which audio signals S.sub.L and S.sub.R are respectively passed through low-pass filters 3 and 4. In such case, in the A/D converter 5, oversampling is performed at a frequency 2f.sub.s which is twice a normal sampling frequency f.sub.s. In such oversampling, the signals S.sub.L and S.sub.R are alternately sampled in the order of S.sub.L, S.sub.R, S.sub.L, S.sub.R, . . . , starting from S.sub.L. Therefore, in order to obtain a sampling rate of 2f.sub.s for each of the two-channel signals, a sampling clock having a frequency of 2f.sub.s .times.2 is supplied to the A/D converter 5.
The 2-channel digital signals obtained from the A/D converter 5 are supplied to a digital low-pass filter 6 (to be hereinafter referred to simply as a "digital filter"). In the digital filter 6, the sampling rate of 2f.sub.s is converted to a normal sampling rate of f.sub.s. Thereafter, the digital signals are supplied to a digital recording and reproducing circuit 7, and are thereby recorded on a magnetic tape.
In a reproducing mode, the L- and R-channel digital signals reproduced from the magnetic tape are supplied from the recording and reproducing circuit 7 to the digital filter 6, and are sampled at the sampling rate of f.sub.s. Thereafter, the digital signals are supplied to a common D/A converter 8. Sampling is started from the L channel at the sampling rate of 2f.sub.s, and the digital signals are alternately oversampled in the order of L, R, L, R, . . . to be converted to analog signals. The analog signals are passed through low-pass filters 9 and 10 to output terminals 11 and 12 and are derived from the latter as original 2-channel signals S.sub.L and S.sub.R.
By reason of the oversampling performed by the A/D converter 5 in the recording mode and by the D/A converter 8 in the reproducing mode, the design of the low-pass filters 3, 4, 9 and 10 can be facilitated. In this connection, if it is assumed that the analog audio signals S.sub.L and S.sub.R have a bandwidth of about 0 to f.sub.s /2, as shown in FIG. 2A, then recording and reproducing signals supplied to and from the recording and reproducing circuit 7 have attenuation characteristics near f.sub.s /2 and 3f.sub.s /2, as shown in FIG. 2B.
For the digital filter 6, there is employed a filter having filter characteristics for attenuating an input within the range of f.sub.s /2 to 3f.sub.s /2, as shown in FIG. 2C. Therefore, each of the low-pass filters 3, 4, 9 and 10 can allow a signal as shown in FIG. 2A to pass therethrough and can fully remove a signal of 3f.sub.s /2 or higher shown on FIG. 2C. For example, the filters 3, 4, 9 and 10 can have a moderate attenuation characteristic curve as indicated by a dotted line in FIG. 2D. For this purpose, design of the low-pass filters 3, 4, 9 and 10 can be facilitated, thus providing practical advantages in respect of signal distortion and cost. In the above case, the oversampling frequency is set to be twice the normal frequency f.sub.s. If the magnification of the oversampling is further increased, the inclination of the characteristic curve of each low-pass filter 3, 4, 9 or 10 can be further decreased.
In the above-mentioned digital recording/reproducing apparatus, it is preferable that L- and R-channel signals be alternately recorded on the magnetic tape without a phase difference therebetween.
In the circuit shown in FIG. 1, in the recording mode, the signals S.sub.L and S.sub.R passing through the low-pass filters 3 and 4 are alternately sampled, in the order of L, R, . . . , starting from the L channel, by the A/D converter 5, and are supplied to the digital filter 6. In this digital filter 6, L- and R-channel signal processing operations are started at the same time. For this reason, the L-channel signal output from the digital filter 6 is phase-delayed relative to the R-channel signal output therefrom, and these signals are recorded on the magnetic tape with a phase difference.
In the reproducing mode, the 2-channel signals which are recorded on the magnetic tape with no phase difference are reproduced, and are simultaneously subjected to signal processing by the digital filter 6. Thereafter, these signals are alternately sampled by the D/A converter 8 in the order of L, R, . . . , starting from the L-channel. As a result, the L-channel signal output from the D/A converter 8 is phase-advanced relative to the R-channel signal output. The 2-channel reproducing signals having a phase difference are supplied to 2-channel loudspeakers, thus impairing the stability and stereoscopic sound effects.
In order to solve the above problems, separate A/D and D/A converters can be provided for each of the 2-channel signals, that is, the L- and R-channel signals. However, the circuit arrangement is thereby complicated and the cost is increased.
Japanese Patent Laid-Open (Kokai) No. 60-242717 discloses a technique for reproducing signals while matching L- and R-channel phases. However, this patent does not describe a recording/reproducing apparatus at all.